1. Field of the Invention
The present invention relates generally to a process for spot-joining stacked metal foils, and more particularly to a process for performing a friction stir spot-welding operation to advantageously realize electrical and mechanical integral joining of a plurality of metal foils stacked or superposed on each other, by using a double-acting type rotary tool.
2. Discussion of Related Art
Metal foils such as aluminum foils and copper foils, which have high electric conductivity, have been used as an electrode such as a current collector of a battery, in a form wherein a plurality of metal foils are stacked or superposed on each other. In a stack of the metal foils which are superposed on each other and which function as the electrode, it is required that the metal foils are fixed together in direct contact with each other, so that the metal foils are electrically and mechanically joined as a unit.
As a conventional process for joining the metal foils stacked on each other as described above, a process using ultrasonic waves has been generally employed. However, a joint strength achieved by this process is undesirably low, giving rise to a problem of failure to achieve stable joining, so that there is a risk of delamination of the stack of the metal foils, due to thermal expansion during charging and discharging of a battery, for example. As a process which solves this problem, a joining process using a friction stir welding operation as a solid-state welding operation has been proposed. Examples of such a process are disclosed in JP-A-2003-126972 and JP-A-2005-103586.
In the process disclosed in JP-A-2003-126972, the friction stir welding operation is performed in an arrangement wherein metal plates having larger thicknesses than metal foils are superposed on respective opposite surfaces of a stack of a plurality of metal foils such that the stack of the metal foils is interposed between the metal plates. In this process, the friction stir welding operation is intended to be performed by using a fixed type tool (rotary tool) with a structure wherein a pin having a predetermined length is provided integrally with a distal end of a cylindrical body which is rotatable about its axis. Accordingly, where a spot-joining operation is performed, a tool hole such as a pin hole (probe hole) is formed and left in a weld due to removal of the welding tool after the welding operation. The existence of the tool hole results in deterioration of the joint strength, so that in JP-A-2003-126972, a linear-joining operation is performed rather than the spot-joining operation, in order to avoid the deterioration of the joint strength. However, where the linear-joining operation is performed, there is a risk that the stacked metal foils are displaced relative to each other due to a movement of the welding tool during the welding operation, unless the metal foils are very firmly fixed together. In order to avoid such a risk, there has been proposed a process in which the metal foils are fixed together by using a bonding agent, but addition of a step of bonding together the metal foils with the bonding agent inevitably results in undesirable increases of a workload and a cost.
On the other hand, JP-A-2005-103586 discloses a process for joining a collective body of a plurality of metal foils by the friction stir welding operation. In this process, the friction stir welding operation is performed while the collective body is fixed by swaging or caulking with a pressure applied in opposite directions perpendicular to upper and lower surfaces of the collective body, or by temporary joining of the collective body by ultrasonic wave welding, cold welding or arc welding. Alternatively, the friction stir welding operation is performed while a reinforcement substrate made of the same metal as the collective body is held in abutting contact with the collective body. However, in this process, too, the linear-joining operation is intended to be performed by using the fixed type tool (rotary tool), so that this process also has the inherent problem as described above with respect to the process of JP-A-2003-126972.